Diffusion transfer photographic processes utilizing silver salts, such as silver halides, etc., are well known to persons skilled in the art. In such photographic processes, a light-sensitive layer of a photographic light-sensitive elements which is prepared by dispersing fine grains of a light-sensitive silver salt such as silver halide in a hydrophilic binder such as gelatin, polyvinyl alcohol, carboxymethyl cellulose, polyvinylpyrrolidone, methyl cellulose, etc., and applying the dispersion to a base such as paper, baryta paper or a base of a high polymer such as polyethylene terephthalate, cellulose diacetate, cellulose triacetate, cellulose nitrate, polycarbonate, polyvinyl chloride, etc., is imagewise exposed to light as a function of incident electromagnetic light rays, and developed by bringing it into contact with a processing solution containing a developer. In such case, exposed silver halide in the light-sensitive layer is reduced (developed) to form nondiffusible silver. At about the same time or thereafter, the light-sensitive element is brought into contact with a water soluble silver complex salt forming agent, by which unexposed silver halide reacts with the water soluble silver complex salt forming agent to form a water soluble silver complex compound. At this time, if an image receiving element having a layer (i.e., an image receiving layer) wherein a substance which becomes a catalyst for a reduction reaction of the above described water soluble silver complex compound (silver precipitation nuclei which are so-called nuclei of physical development) is dispersed in a hydrophilic binder and is allowed to contact the above described light-sensitive layer, the silver complex compound formed in the light-sensitive layer diffuses by means of the processing solution from the light-sensitive layer into the image receiving layer, where it is reduced to form silver by the function of the nuclei of development. That is, silver images are formed on the image receiving face as if the images were transferred from the light-sensitive layer to the image receiving layer. Therefore, the above described photographic process is called the silver salt diffusion transfer photographic process.
Image receiving elements for the diffusion transfer photographic process which contain a silver precipitant in a matrix material capable of permeating alkali processing compositions have been known and described, for example, in British Pat. No. 1,149,921.
In the technical field of silver salt diffusion transfer photography, various studies concerning image receiving elements used for such photography have been hitherto conducted. For example, metal sulfides, metal selenides, and heavy metal or noble metal colloids of low solubility in water are generally used as silver precipitation nuclei and it is desirable that these silver precipitation nuclei for the image receiving elements have high activity. For example, U.S. Pat. No. 2,698,237 has described a process for obtaining silver precipitation nuclei having high activity which comprises blending a water soluble metal salt with a water soluble sulfide in fine silica to form a precipitate of water insoluble metal sulfide. Further, Japanese Patent Publication No. 32754/69 has described image receiving elements which are prepared by a process comprising incorporating a silver precipitation nucleus substance in an alkali impermeable polymer by a vacuum evaporation process, dissolving said polymer in a solvent, applying the resulting solution to a base, drying it, and thereafter subjecting the surface layer of the polymer layer to chemical processing such as hydrolysis, etc., so as to be alkali permeable.
Further, Japanese Patent Application (OPI) No. 73150/73 (The term "OPI" as used herein refers to a "published unexamined Japanese Patent Application") has disclosed image receiving elements for the silver salt transfer process which are prepared by hydrolyzing a cellulose ester layer and incorporating silver precipitation nuclei in the hydrolyzed layer simultaneously with or after hydrolysis. However, there is a problem in that silver images formed on resulting image receiving elements easily discolor or fade during preservation.
As a way of overcoming this problem, Japanese Patent Publication No. 5392/71, U.S. Pat. No. 3,533,789 and British Pat. No. 1,164,642 have disclosed a process which comprises applying a solution of a water soluble polymer containing an alkali neutralizing component to the surface of the obtained silver images. However, in this process, a long period of time is required for completely drying the surface to which an aqueous solution of the polymer is applied, and the prints can not be placed on top of one another because the surface is soft and sticky, and fingerprints and dust often adhere to the surface. In addition, application of such a solution to the silver images is troublesome.
Japanese Patent Publication No. 44418/81, corresponding to U.S. Pat. No. 3,607,269, has disclosed image receiving materials for a silver salt diffusion transfer process which comprises a base, a cellulose ester, polyvinyl ester, or polyvinyl acetate layer I containing a diffusible chemical suitable for modifying the photographic characteristics of silver transfer images, which is hydrolyzable and becomes alkali permeable by hydrolysis, provided on the base, and a regenerated cellulose layer II containing silver precipitation nuclei provided on said layer I, wherein the layer I does not contain silver precipitation nuclei and the layer II does not contain the above described diffusible chemical. As the diffusible chemical, organic mercapto compounds have been described.
According to the above described Japanese Patent Publication No. 44418/81, at column 2, lines 25-37, it has been described that the above described diffusible chemical is placed below the image forming layer prior to carrying out diffusion transfer processing, and a toning agent and a stabilizer are released from the lower layer by extraction during the diffusion transfer processing, and, consequently, the effect by the chemical is enhanced during the diffusion transfer processing at column 2, lines 25-37. Further in the above described Japanese Patent Publication, at column 6, line 44 to column 7, line 19, it has been described that these layers I and II are not independent or separated layers, but are formed as a single continuous layer wherein a part in the depth direction is modified, and that, in case of producing these layers by a plurality of continuous applications, a common solvent is used to form a single continuous layer without forming interfaces between layers. Accordingly, this process is understood as a characteristic technique for obtaining the above described effect. According to this process, the toning agent is at least present in the layer II in the very initial stage of development and it diffuses from the layer I into the layer II during development to act as a toning agent. As is well known to persons skilled in the art, the toning agent has a function of changing color of images, which acts in the process of forming development silver to provide an influence upon the surface state of the formed development silver or other optical characteristics. Accordingly, it is naturally ineffective if it does not diffuse from the lower layer during formation of development silver to act with it.
However, the process described in Japanese Patent Publication No. 44418/81 involves certain problems in practice, and it is difficult to realize as a practical process. Firstly, in the process for producing the image receiving materials, if the diffusible chemical is incorporated in only layer I, layer I is swollen by the solvent for the layer II in case of applying the layer II and the diffusible chemical contained in the layer I diffuses into the layer II and it is nearly uniformly redistributed in the layer I and the layer II, because exemplified mercapto substituted compounds such as 1-phenyl-5-mercaptotetrazole or imidazolidine-thione, etc., are very soluble in the organic solvent used for containing. Secondly, although the diffusible chemical in layer II is removed by dissolving in a hydrolyzing solution by the hydrolysis processing, the diffusible chemical tends to diffuse from layer I into layer II before the image receiving materials produced can be used photographically by a user, whereby the photographic performance deteriorate. Further, if the amount of the chemical added to layer I is reduced in order to avoid such problems satisfactory modification of photographic characteristics of transfer images is not obtained. Since layer I and layer II are composed of very analogous components such that an interface is not formed, and, particularly since mutual diffusion of substances between layers I and II easily occurs, the abovedescribed problems are very difficult to avoid.